Method for producing a ceramic electronic part

ABSTRACT

A ceramic electronic part includes a plurality of internal electrodes arranged in a ceramic sintered compact, superimposed via a ceramic layer. The free ends of the internal electrodes are formed to have a wedge-like cross-sectional shape, with the length L of the wedge and the internal thickness t of the electrode at the base of the wedge satisfying the relationship L&gt;2t so that there is no risk of generating inter-layer peel-off or delamination in the ceramic layers.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of U.S. patent application Ser. No.09/361,763, Jul. 27, 1999, in the name of Tatsuo Haratani et al.entitled “CERAMIC ELECTRONIC PART.”

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a ceramic electronic part, suchas a laminated capacitor, and a method for producing the same. Morespecifically, it relates to a ceramic electronic part with an improvedshape for the lateral edges of its internal electrodes and a method forproducing the same.

[0004] 2. Description of the Related Art

[0005]FIG. 6 is a cross-sectional view showing an embodiment of aconventional laminated capacitor. The laminated capacitor 51 has aceramic sintered compact 52 comprising dielectric ceramics. A pluralityof internal electrodes 53 a to 53 d are formed in the ceramic sinteredcompact 52 such that they are superimposed in the thickness directionvia respective ceramic layers. The internal electrodes 53 a, 53 c extendto the end face 52 a, and the internal electrodes 53 b, 53 d extend tothe end face 52 b. External electrodes 54, 55 cover the end faces 52 a,52 b, respectively.

[0006] The ceramic sintered compact 52 to be used in the above-mentionedlaminated capacitor 51 is produced by the process described below. Aplurality of ceramic green sheets having internal electrodes 53 a to 53d printed thereon are laminated together with an optional number ofunprinted ceramic green sheets located on the upper and lower surfacesof the compact so as to obtain a laminated product. A conductive pasteis used to form the internal electrodes. After the laminated product hasbeen pressed in the thickness direction, it is baked.

[0007] Because the internal electrodes 53 a to 53 d are formed by bakingthe printed conductive paste, they have a substantially homogeneousthickness. The tip (one lateral edge) of the internal electrode 53 a,that is, the part shown in the circle in FIG. 6, is shown in an enlargedview of FIG. 7A.

[0008] As apparent from FIG. 7A, the internal electrode 53 a has asubstantially homogeneous thickness to the tip. As shown in FIG. 7B, insome cases the tip of the internal electrode 53 a may have a slightroundness.

[0009] However, due to the above-mentioned shape of the internalelectrode, when the ceramic sintered compact 52 is formed, peel-offamong the ceramic layers or the phenomenon called delamination sometimesoccurs. This can be attributed to the fact that when the laminatedproduct is pressed prior to baking, portions where the internalelectrodes exist are compressed strongly in the thickness direction soas to have a higher density, whereas portions where the internalelectrodes do not exist are insufficiently compressed. As a result, thedensity of the portions B, C and D shown in FIG. 7A before baking varyin the order B>D>C. This variation causes delamination or inter-layerpeel-off in the sintered compact. In particular, in the case of aninternal electrode with a thickness of 3 μm or more (used for highreliability), this phenomena has been remarkable.

[0010] A method for alleviating the reduction in density in the partshown as C in FIG. 7A is disclosed in Japanese Unexamined PatentPublication No. 8-58259. This publication proposes a method of printinga conductive paste, using a special screen printing plate, wherein thethickness of the internal electrode in the vicinity of its outer rim isthinner than at its other parts. The cross-sectional shape of aninternal electrode formed according to this method is shown in FIG. 8.In the vicinity of the outer rim of the internal electrode 56 shown inFIG. 8, the internal electrode 56 is thinner at part 56 b as a result ofthe presence of step 56 a.

[0011] However, even in the sintered compact obtained by the methoddisclosed in the official gazette of Japanese Unexamined PatentPublication No. 8-58259, delamination and inter-layer peel-off is notsatisfactorily prevented, and further improvement is desired.

SUMMARY OF THE INVENTION

[0012] An object of the present invention is to provide a ceramicelectronic part such as a laminated capacitor, capable of effectivelyminimizing delamination and inter-layer peel-off phenomenon by improvingthe internal electrode shape, and a production method thereof.

[0013] A first aspect of the present invention is a ceramic electronicpart comprising a ceramic sintered compact and a plurality of internalelectrodes in the ceramic sintered compact, at least one lateral edge ofat least one of the internal electrodes having a wedge-likecross-sectional shape, the length L of the wedge in the thickness t ofthe internal electrode at the base of the wedge satisfying therelationship L>2t.

[0014] Since the lateral edge of the internal electrode is formed tohave a wedge-like cross-sectional shape, with the length L of the wedgeand the thickness of the internal electrode at the base of the wedgesatisfying the relationship L>2t, delamination or the inter-layerpeel-off can be effectively restrained in the ceramic sintered compact.In particular, even in the case the thickness of the internal electrodesis increased to 3 μm or more, since the lateral edge has theabove-mentioned wedge-like shape, generation of delamination can beeffectively restrained. Therefore, a ceramic electronic part with anexcellent reliability can be provided as the production yield can beimproved.

[0015] In a second aspect of the present invention, the internalelectrodes and the ceramic sintered compact are selected such that thecontraction ratio of the ceramic material forming the sintered compactby baking is larger than the contraction ratio of the material formingthe internal electrodes. Since the materials are selected such that thecontraction ratio of the ceramics by baking is larger than thecontraction ratio of the internal electrodes, the lateral edges of theinternal electrodes can securely have the above-mentioned wedge-likecross-sectional shape after baking.

[0016] In a third aspect of the present invention, the lateral edges ofthe internal electrodes are formed to have a wedge-like cross-sectionalshape, with the length L of the wedge and the thickness of the internalelectrode t at the base part of the wedge satisfies L>2t. Since thelateral edges of the internal electrodes are formed to have a wedge-likecross-sectional shape, and to satisfy the relationship L>2t,delamination can securely be prevented also at the side face of theceramic sintered compact.

[0017] In a fourth aspect of the present invention, the ceramic sinteredcompact is made from dielectric ceramics, and the plurality of theinternal electrodes are laminated via a ceramic sintered compact layerin the thickness direction of the ceramic sintered compact so as toprovide a laminated capacitor. Since the ceramic sintered compact ismade from dielectric ceramics, and the plurality of the internalelectrodes are laminated via a ceramic sintered compact layer, alaminated capacitor with an excellent reliability without the risk ofgenerating delamination in the ceramic sintered compact according to thepresent invention can be provided with a high yield.

[0018] In a fifth aspect of the present invention, the thickness of theinternal electrodes is in the range of 3 to 20 μm. With a thickness of 3μm or greater, the internal electrodes themselves and the connectionbetween the internal electrodes and the external electrodes can be morereliable, and the current capacity can be larger compared with the caseof less than 3 μm. As to the upper limit of the internal electrodethickness, 20 μm or less is preferable as mentioned above in view ofhaving a small size.

[0019] According to the fifth aspect of a ceramic electronic part of thepresent invention, since the thickness of the internal electrodes is 3μm or greater, a laminated electronic part having a high reliability inthe internal electrodes themselves and the connection between theinternal electrodes and the external electrodes and a large currentcapacity can be obtained.

[0020] A sixth aspect of the present invention is a production method ofa ceramic electronic part with a plurality of internal electrodesarranged in a ceramic sintered compact, comprising laminating aplurality of ceramic green sheets one on top of the other, at least someof the ceramic green sheets having conductive paste on a surface thereofsuch that the conductive paste is located between two adjacent ceramicgreen sheets, baking the laminated product to obtain the electronicpart, the contraction ratio of the ceramic material forming the ceramicgreen sheets being greater than the contraction ratio of the conductivepaste.

[0021] Since ceramics and a conductive paste with the contraction ratioof the ceramics by baking which is larger than the contraction ratio ofthe internal electrodes are used as the material of the ceramic greensheets and the conductive paste in obtaining a ceramic sintered compactafter obtaining a laminated product by laminating a plurality of ceramicgreen sheets with a conductive paste for providing the internalelectrodes printed and unprinted ceramic green sheets by baking thelaminated product, a ceramic electronic part with the internalelectrodes having a wedge-like cross-sectional shape according to thepresent invention can be provided easily and securely.

[0022] In particular, according to the production method of the fifthaspect, a ceramic electronic part according to the present invention canbe provided in a process which is otherwise the same as the conventionalproduction method by selecting the material for providing the internalelectrodes and the conductive paste without the need of a jig such as aspecial screen printing plate, or the like. Therefore, a high reliableceramic electronic part can be provided without the increase of theceramic electronic part cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For the purpose of illustrating the invention, there is shown inthe drawing several forms which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangements and instrumentalities shown.

[0024]FIG. 1A is a side cross-sectional view of a laminated capacitor asa ceramic electronic part according to a first embodiment of the presentinvention.

[0025]FIG. 1B is an enlarged partial cross-sectional view of the partshown by the circle E in FIG. 1A.

[0026]FIG. 2 is a cross-sectional view taken on the line 2-2 of FIG. 1.

[0027]FIG. 3 is a schematic cross-sectional view for explaining theoperation of a ceramic electronic part according to the presentinvention and a ceramic laminated product before baking.

[0028]FIG. 4 is a schematic partial cross-sectional view for explainingthe sintered compact obtained by baking the laminated product shown inFIG. 3.

[0029]FIG. 5 is a microscope photograph for explaining the shape of thelateral edge of the internal electrode of the laminated capacitorobtained in a second embodiment of the invention.

[0030]FIG. 6 is a cross-sectional view showing an example of aconventional laminated capacitor.

[0031]FIGS. 7A and 7B are partial enlarged cross-sectional views forexplaining the shape of the lateral edge of the internal electrode inthe conventional laminated capacitor.

[0032]FIG. 8 is a partial enlarged cross-sectional view for explainingthe shape of the lateral edge of the internal electrode of aconventional laminated capacitor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033]FIG. 1A is a side cross-sectional view of a ceramic electronicpart according to a first embodiment of the present invention, and FIG.1B is an enlarged partially notched cross-sectional view of the partshown by the circle E in FIG. 1A. In this embodiment, the ceramicelectronic part is a capacitor.

[0034] A laminated capacitor 1 has a ceramic sintered compact 2comprising dielectric ceramics. A plurality of internal electrodes 3 to6 are formed in the ceramic sintered compact 2 such that they aresuperimposed in the thickness direction via respective ceramic layers.The internal electrodes 3, 5 extend to the end face 2 a of the ceramicsintered compact 2, and the internal electrodes 4, 6 extend to the endface 2 b. External electrodes 7, 8 are formed so as to cover the endfaces 2 a, 2 b, respectively, with the result that external electrode 7is electrically coupled to internal electrodes 3 and 5 and externalelectrode 8 is electrically coupled to internal electrodes 4 and 6.

[0035] The outer lateral edges W (sometimes referred to herein as the“rims” or “free edges”) of the internal electrodes 3 to 6, except theedges extending to the end faces 2 a, 2 b, have a wedge-likecross-sectional shape best shown in FIG. 1B. The shape of the edges Wpreferably satisfy the relationship L>2t, wherein L is the length of thewedge (FIG. 1B) and t is the thickness of the internal electrode at thebase of the wedge. As shown in the experimental results discussed below,the incidence of delamination and inter-layer peel-off is effectivelyrestrained when this ratio is met.

[0036] In order to realize a wedge-like cross-sectional shape satisfyingthe relationship L>2t, the materials used for the internal electrodesand the ceramic sintered compact may be selected such that thecontraction ratio of the ceramics during baking is larger than thecontraction ratio of the internal electrodes during baking. This will beexplained with reference to FIGS. 3 and 4.

[0037]FIG. 3 schematically illustrates the relationship between theinternal electrodes and the ceramic green sheets in the ceramiclaminated product before sintering. The laminated product 11 has ceramicgreen sheets 12 to 14 laminated together with the internal electrodes 3,4 located between them. The internal electrodes 3, 4 are formed byprinting a conductive paste on the upper surface of the ceramic greensheets 13, 14, respectively.

[0038] If the laminated product 11 is compressed in its thicknessdirection (vertically in FIG. 3), the laminated product can besufficiently compressed in the areas where the internal electrodes 3, 4are laminated. However, a sufficient compression is not applied in theareas indicated by arrows G and H in FIG. 3. Therefore, a portion of theceramic material having a low density exists outside of the tips 3 a, 4a of the internal electrodes 3, 4.

[0039] However, when the laminated product 11 is baked, the contractionratio of the ceramics is larger than the contraction ratio of theinternal electrodes 3, 4. Therefore, the ceramics expand as shown inFIG. 4 to deform the tips 3 a, 4 a of the internal electrodes 3, 4 tohave a wedge-like cross-sectional shape. As a result, the ceramics aresintered densely in the vicinity of the tips 3 a, 4 a of the internalelectrodes 3, 4 so that the occurrence of inter-layer peel-off anddelamination is reduced.

[0040] There are many ways to achieve the foregoing contraction ratios.By way of example, and not limitation, two other methods can be used:(1) using a conductive paste with a low ratio of a binder to bescattered after baking and a high ratio of metal powders for theinternal electrodes, and (2) adding a high melting point such as Ni, Moand W to a conductive paste which is used to form the internalelectrodes.

[0041] One non-limiting example of the first method uses a lowtemperature sintering ceramics (CaZrO₃+glass material) and a conductivepaste having about 2 to 5% by weight of an organic binder contained withrespect to 100% by weight of metal powders. One non-limiting example ofthe second method adds a high melting point metal into the internalelectrodes. For example, if low temperature sintering ceramic(CaZrO₃+glass material) is used as the ceramic material and a conductivepaste mainly containing Cu with a 1083° C. melting point is used, about0.5 to 10% by weight of at least one element selected from a group ofelements having a comparatively high melting point with respect to Cu,such as Ni, Mo, W, or the like, can be added with respect to 100% byweight of Cu.

[0042] The composition of the internal electrodes as mentioned above isto be adjusted according to the contraction ratio of the ceramics to beused, and thus any appropriate composition can be used.

[0043] A production method of a ceramic electronic part according to thepresent invention, and the effects of the present invention, will beexplained hereinafter with respect to concrete experimental examples.

EXPERIMENTAL EXAMPLES ACCORDING TO THE INVENTION Experimental Example 1

[0044] In order to obtain a ceramic sintered compact 2, a rectangularceramic green sheet was formed with a ceramic slurry mainly containinglow temperature sintering ceramics (CaZrO₃+glass material) powders. Inorder to form the internal electrodes 3 to 6 on the ceramic green sheet,a conductive paste with a composition ratio of 100% by weight of Cupowders having a 1.0 μm average particle size and 3.0% by weight of anorganic binder was screen printed onto the ceramic green sheets.Thereafter, a plurality of the ceramic green sheets on which theconductive paste was printed were laminated together, and unprintedceramic green sheets were laminated above and below the foregoing groupof sheets. The entire laminate was pressed in the thickness direction toobtain a laminated product. The laminated product was baked at 1000° C.so as to obtain a ceramic sintered compact 2 with a 1.6×0.8×0.8 mm size.The number of laminations of internal electrodes was 4.

[0045] By applying a conductive paste onto opposite end faces of theobtained ceramic sintered compact 2 and baking, external electrodes 7, 8were formed so as to obtain a laminated capacitor 1. The formation ofthe external electrodes 7, 8 can be conducted in the same manner asbaking of the laminated product.

[0046] The final thickness of the internal electrodes after baking inthe laminated capacitor of this embodiment was 3 μm. That is, t=3 μm.From the observation of the lateral edges of the internal electrodesafter cutting the obtained sintered compact, it was determined that thelateral edges had a wedge-like shape and L=10 μm.

Experimental Example 2

[0047] Using the same process described above, a second laminatedcapacitor with a 10 μm final thickness of the center part of theinternal electrodes was produced. From the observation of the lateraledges of the internal electrode after cutting the obtained sinteredcompact, it was determined that the lateral edges of the internalelectrodes had a wedge-like shape as shown in the microscope photographof FIG. 5. In this case, the dimensions of the wedge-like shape weret=10 μm and L=25 μm.

COMPARATIVE EXPERIMENTAL EXAMPLES

[0048] Several comparative embodiments 1, 2, 3, 4 were prepared tocompare the attributes of the present invention to other structures.

Comparative Example 1

[0049] A laminated capacitor was obtained in the same manner as in theExperimental Example 1 except that the internal electrodes with acomposition ratio of 10% by weight of an organic binder with respect to100% by weight of Cu powders having a 1.0 μm average particle size wereused. In this case, the final internal electrode thickness was 3 μm, andfrom the observation of the lateral edges of the internal electrodesafter cutting the obtained sintered compact, it was determined that thethickness of the internal electrodes was about 3 μm throughout,including at its outer lateral edges.

Comparative Example 2

[0050] A laminated capacitor was obtained in the same manner as in theComparative Example 1 except that the thickness of the internalelectrodes was 10 μm. The obtained sintered compact was cut to observethe lateral edges of the internal electrodes. It was determined that thethickness of the internal electrodes was substantially equal to 10 μmthroughout, including at its outer lateral edges.

Comparative Example 3

[0051] A laminated capacitor was obtained in the same manner as in theExperimental Example 1 except that the internal electrodes were printedaccording to the process described in Japanese Unexamined PatentPublication No. 8-58259. The lateral edges of the internal electrode hadthe cross-sectional shape shown in FIG. 8.

Comparative Example 4

[0052] A laminated capacitor was obtained in the same manner as in theComparative Example 3 except that the thickness of the internalelectrodes was changed to be 10 μm. In this laminated capacitor, thecross-sectional shape of the lateral edges of the internal electrode wasthe same as the internal electrode 56 shown in FIG. 8.

Comparative Example 5

[0053] A laminated capacitor was produced in the same manner as in theExperimental Example 1 except that a conductive paste with a compositionof 7.0% by weight of an organic binder with respect to 100% by weight ofCu powders having a 1.0 μm average particle size were used as thematerial comprising the internal electrodes. From the observation of thelateral edges of the internal electrodes after cutting the obtainedsintered compact of the laminated capacitor, it was determined that t=3μm, and L=3 μm. That is, L<2t.

Comparative Example 6

[0054] A laminated capacitor was produced in the same manner as in theExperimental Example 1 except that a conductive paste with a compositionof 7.0% by weight of an organic binder with respect to 100% by weight ofCu powders having a 1.0 μm average particle size were used. From theobservation of the lateral edges of the internal electrodes aftercutting the obtained sintered compact, it was determined that it had awedge-like shape. However, t=10 μm, and L=5 μm. That is, L<2t.

[0055] The delamination generation ratios of the sintered compact in the10 pieces of the laminated capacitor of the Experimental Examples 1, 2and the Comparative Examples 1 to 6 obtained as mentioned above areshown in Table 1. TABLE 1 Delamination Generation Ratio (in % ofdelamination Sample occurrences) Experimental Example 1  0% ExperimentalExample 2  0% Comparative Example 1 20% Comparative Example 2 100% Comparative Example 3  5% Comparative Example 4 60% Comparative Example5 10% Comparative Example 6 80%

[0056] As apparent from Table 1, the delamination generation ratio ishigh in the Comparative Examples 1, 2 with the thickness of the internalelectrode at the outer lateral edges being substantially the same asthat at the center of the internal electrode. In particular, with athick internal electrode thickness of 10 μm, delamination was generatedin the largest percentage of sintered compacts.

[0057] Similarly, in the Comparative Examples 3, 4, although thedelamination generation ratio is lower than that in the ComparativeExamples 1, 2, delamination generation was not prevented. Furthermore,with an internal electrode thickness of 10 μm, delamination wasgenerated with a considerable rate.

[0058] Moreover, in the Comparative Examples 5, 6, probably becauseL<2t, the delamination generation ratio was 10% and 80%, respectively.

[0059] On the other hand, in the Experimental Examples 1, 2, there wereno observed occurrences of delamination generation regardless of thethickness of the internal electrodes.

[0060] Although explanation has been given in the above-mentionedembodiments, the present invention can be adopted similarly in variousceramic electronic parts such as a laminated inductor, a laminatedvarister, a laminated piezoelectric ceramic part, or the like with thesame effect of effectively preventing delamination in a ceramic sinteredcompact.

[0061] The present invention may be embodied in other specific formswithout departing from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

What is claimed is:
 1. A method for producing an electronic part havinga plurality of internal electrodes, said method comprising: laminating aplurality of ceramic green sheets one on top of the other, at least someof the ceramic green sheets having conductive paste on a surface thereofsuch that the conductive paste is located between two adjacent ceramicgreen sheets, baking the laminated product to obtain the electronicpart, the contraction ratio of the ceramic material forming the ceramicgreen sheets being greater than the contraction ratio of the conductivepaste.